1. Field of the Invention
The present invention relates to thermoplastic bags which are provided with integral handle members on opposite sides of the bag mouth and, more particularly such thermoplastic bags having an additional thermoplastic reinforcement in the handle area.
2. Description of the Prior Art
In the past, bags which were characterized by having carrying handles thereon were constructed using separate handle elements, distinct from the bag structure itself. Such handle elements were individually fed, during bag manufacture, for attachment adjacent to the open mouth portion of the bag. The manufacturing operation to produce such prior art structures with the separate process step of supplying handle elements, and applying them to the bag, is quite cumbersome and uneconomical.
More recently, however, bag structures have been developed, see for example, U.S. Pat. Nos. 4,085,822; 3,352,411; and 3,180,557; and Belgian Pat. No. 862,069, the disclosures of which are incorporated herein by reference, wherein the bags are formed so that the handle carrying elements are formed as an integral part of the bag structure itself. That is, the handles are actually an extension of the bag walls. An example of such a bag structure is one that is constructed from a flattened tube or a flattened side edge gusseted tube which is sealed at both ends. A flattened end portion of such a tube is cut off to form an open mouth bag having handles on either side thereof. Conversely, such a bag may be formed by folding a piece of the thermoplastic material on itself, the bottom fold line constituting the bottom part of the bag and heat sealing the upper edge and side wall parts of the bag together. Next, a U-shaped cutout is made in the upper portion of the bag to provide an opening or entrance for the introduction of goods to be packaged. The opposite edges of the upper portion of the bag structure immediately adjacent to the cutout area form loops which may be used to carry such bag structures when they are loaded, i.e., double ply thickness, by virtue of the presence of the re-entrant or gusset fold in the loop handle members.
These prior art bag structures present difficulties for the end user, particularly since, for reasons of economy, such structures are usually produced from extremely thin thermoplastic material, i.e., on the order of about 1.0 mil or less. There is a tendency for such bags to fail in the handle area where, when the loaded bags are being carried, there is a tendency for the stress forces to concentrate. Prior art bag structures of the type hereinabove described are usually formed from low-density polyethylene material.
It has been found that the use of a mono-layer homogeneous composition for both the handle and the body portion of the hereinabove described bags requires necessarily that the resin and thickness choice fulfill the handle tensile strength requirements and body tear and puncture requirements simultaneously. Obviously, such an arrangement is not the most economical utilization of available resin materials. Generally speaking, a single specialty resin which would accomplish all three of these requirements, when produced in the form of a relatively thin film structure, would be economically prohibitive in the case of the manufacture of the present disposable bag structures.
In general, low density polyethylene has certain physical characteristics which make its employment as a body film desirable for the handle bag constructions of the present invention. In particular, such advantages include its low cost, high puncture resistance, and its resistance to splitting and tearing while under load stress. Conversely, however, as hereinabove noted, low density polyethylene exhibits unsatisfactorily low load bearing ability in the handle regions. The relative low tensile yield properties of low density polyethylene account in part for this deficiency. To increase handle strength, it has been proposed, in application Ser. No. 086,374, filed Oct. 18, 1979, discussed above, to reinforce the handles of the bags with reinforcement films which are laminated to the bag and extend from a top to a bottom edge thereof.
The reinforcement film is specifically designed to improve the overall tensile yield and strength characteristics of the bag in the handle. Materials which can be used for the reinforcement film include fractional melt index low-density polyethylene, and other thermoplastic materials, such as linear low-density polyethylene, polypropylene, high-density polyethylene, and thermoplastic films which are highly oriented in the machine direction, including film materials which have been cold drawn, i.e., stretched, at ambient temperatures. In addition to cold drawing techniques to achieve the requisite strength characteristics for the reinforcing handle film, the reinforcement film may also be melted oriented during extrusion. In the case of high-density polyethylene, such melt orientation may be achieved during tubular extrusion thereof by employing low blow-up ratios on the order of about 2:1. A range of such low blow-up ratios which may be employed to achieve melt orientation of high density polyethylene encompasses from about 1:1 up to about 2.5:1. It has been found that the employment of oriented handle laminating materials in the bag handle area result in improvements in both tensile strength and the yield strength of the film.
Although the reinforcement films employed provide a considerable increase in handle strength, they extend to areas of the bag where reinforcement is not required, thereby wasting resin and adding to overall bag cost.
In addition, during bag construction, heat sealing is employed at areas defining the top and bottom edges of the bag. This heat sealing occurs along a bag edge which has thinner areas defined by the bag film alone and thicker areas defined by gussets in the bag film and by the lamination of the bag film and the reinforcement film. Heat sealing across the bag edge becomes difficult because of the different material thickness. An optimal temperature and dwell time for heat sealing through one thickness will not be the optimal conditions for heat sealing through the other. As a result, a compromise temperature and dwell time must be used which is not optimal for either of the material thicknesses. The heat sealing problem can be further aggrevated if different polymers are used for the bag body and reinforcement film.